Next-generation non-volatile memory devices with lower power consumption and higher degree of integration than flash memory devices are being studied. Such next-generation non-volatile memory devices include phase-change random access memory (PRAM) that uses state changes of a phase change material such as chalcogenide alloys, magnetic random access memory (MRAM) that uses resistance changes in a magnetic tunnel junction (MTJ) depending on the magnetization state of a ferromagnetic material, ferroelectric random access memory (FRAM) that uses polarization of a ferroelectric material, resistance-change random access memory (ReRAM) that uses resistance changes in a variable resistance material, and the like.
Examples of MRAM include a spin-transfer torque magnetic random access memory (STT-MRAM) device that inverts magnetization using a spin-transfer torque (STT) phenomenon generated by electron injection and discriminates a resistance difference before and after magnetization inversion. The STT-MRAM device includes a magnetic tunnel junction, which consists of a pinned layer and a free layer, each formed of a ferromagnetic material, and a tunnel barrier disposed therebetween. In the magnetic tunnel junction, when the magnetization directions of the free layer and the pinned layer are the same (that is, parallel), current flow is easy and consequently the magnetic tunnel junction is in a low resistance state. On the other hand, when the magnetization directions are different (that is, antiparallel), current is reduced and consequently the magnetic tunnel junction is in a high resistance state. In addition, in the magnetic tunnel junction, the magnetization directions must change only in the direction perpendicular to a substrate. Therefore, the free layer and the pinned layer must have perpendicular magnetization values. When the perpendicular magnetization values are symmetrical with respect to 0 according to the intensity and direction of a magnetic field, and a squareness (S) shape becomes clear (S=1), perpendicular magnetic anisotropy (PMA) is considered to be excellent. The STT-MRAM device is theoretically capable of cycling more than 1015 times and can be switched at a high speed of about a few nanoseconds (ns). In particular, a perpendicular magnetization type STT-MRAM device is advantageous in that there is no theoretical scaling limit, and as scaling progresses, the current density of driving current may be lowered. Therefore, the perpendicular magnetization type STT-MRAM device has been actively studied as a next-generation memory device that may replace DRAM devices. An example of the STT-MRAM device is disclosed in Korean Patent No. 10-1040163.
In the STT-MRAM device, a seed layer is formed on the lower part of the free layer, a separation layer is formed on the upper part of the pinned layer, and synthetic antiferromagnetic (SyAF) layers and an upper electrode are formed on the upper part of the separation layer. In addition, in the STT-MRAM device, a silicon oxide film is formed on a silicon substrate, and then the seed layer and a magnetic tunnel junction are formed thereon. In addition, a selection element such as a transistor may be formed on the silicon substrate, and the silicon oxide film may be formed so as to cover the selection element. Therefore, the STT-MRAM device has a laminated structure in which a silicon oxide film, a seed layer, a free layer, a tunnel barrier, a pinned layer, a separation layer, SyAF layers, and an upper electrode are formed on a silicon substrate on which a selection element is formed. In this case, the separation layer and a capping layer are formed using tantalum (Ta), and the SyAF layers have a structure in which a lower magnetic layer and an upper magnetic layer, in which a magnetic metal and a non-magnetic metal are alternately laminated, are formed, and a non-magnetic layer is formed therebetween. That is, on the substrate, the magnetic tunnel junction is formed on the lower part and the SyAF layers are formed on the upper part.
However, since SyAF layers with a face-centered cubic (FCC) (111) structure are formed on the upper side of the magnetic tunnel junction in which texturing is performed in the body-centered cubic (BCC) (100) direction, the FCC (111) structure diffuses into the magnetic tunnel junction when the SyAF layers are formed, which may deteriorate the BCC (100) crystal. That is, when the SyAF layers are formed, some of a material forming the SyAF layers may diffuse into the magnetic tunnel junction, thus deteriorating the crystallinity of the magnetic tunnel junction. Therefore, the magnetization direction of the magnetic tunnel junction may not be rapidly changed, such that the operation speed of a memory may be lowered or the memory may not operate.
To overcome this problem, SyAF layers may be first formed on a substrate, and then a magnetic tunnel junction may be formed thereon. In this case, the SyAF layers are formed on a seed layer, and the seed layer is formed using any one of ruthenium (Ru), hafnium (Hf), and tantalum (Ta). When the magnetic tunnel junction is formed on the SyAF layers, the SyAF layers must grow in the FCC (111) direction to fix the pinned layer of the magnetic tunnel junction in the perpendicular direction. However, Ru, Hf, and Ta, which are generally used to form a seed layer, are not suitable for growth of the SyAF layers in the FCC (111) direction, and thus a high magnetoresistance (MR) ratio may not be generated in the magnetic tunnel junction.
In addition, after a memory device is formed in this manner, a passivation process and a metal wiring process are performed at a temperature of about 400° C. However, Ta used to form a separation layer and a capping layer diffuses into the magnetic tunnel junction, which lowers the perpendicular magnetic anisotropy of the magnetic tunnel junction. As a result, a high magnetoresistance ratio may not be generated.
In addition, the SyAF layers generally have a structure in which a first magnetic layer having a multilayer structure, a non-magnetic layer, and a second magnetic layer having a multilayer structure are laminated. For example, the first magnetic layer is formed by laminating cobalt (Co) and platinum (Pt) at least six times, and the second magnetic layer is formed by laminating Co and Pt at least three times. Since the first and second magnetic layers are each formed in a multilayer structure, the memory device becomes thick. In addition, rare-earth elements are often used to form the first and second magnetic layers, and thus processing costs increase.